(re-)add possibility to include or check 'external' references

[kosloot]

The "External" implementation has been simplified in commit af1817f

In fact NO resolving is done anymore now. This also implies that NO CHECKING is done on those external files.
It would be nice, and really simple to implement, to have a way to check the sanity.

Like adding a new 'mode' for the documents: "check-externals".
Even more easy to do is implementing a "resolve-externals" mode, which is nothing else than the old include="yes"

WARNING: for the "check-externals" variant it is important to be sure that no the external document doesn't introduce new sets or annotations in the 'master' document. (is that even possible?)

[kosloot]

@proycon any ideas about such an enhancement? how does FoLiApy (foliavalidator) handle checking of externals?
And how about:

for the "check-externals" variant it is important to be sure that no the external document doesn't introduce new sets or annotations in the 'master' document. (is that even possible?)